Cardiac pacemaker lead systems fulfill two functions. The first function is to provide an electrical conduit by which a pacemaker output pulse is delivered to stimulate the local tissue adjacent to the distal tip of the lead. The second function is to sense local, intrinsic cardiac electrical activity that takes place adjacent to the distal tip of the lead.
The advantages of providing pacing therapies to both the right and left heart chambers are well established. For example, in four chamber pacing systems, four pacing leads, typically bipolar leads, are positioned for both pacing and sensing of the respective heart chambers. To provide right side pacing and sensing, leads are implanted directly in the right atrium and/or right ventricle. To provide left side stimulation and sensing, leads are transvenously implanted in the coronary sinus region, for example, in a vein such as the great vein, the left posterior ventricular (LPV) vein, or other coronary veins, proximate the left ventricle of the heart. Such placement avoids the risks associated with implanting a lead directly within the left ventricle which can increase the potential for the formation of blood clots which may become dislodged and then carried to the brain where even a small embolism could cause a stroke. (As used herein, the phrase “coronary sinus region” refers to the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other coronary vein accessible by way of the coronary sinus.)
One of the problems with cardiac pacing and sensing systems is their inability to suppress far-field electrical signals. These signals are generated by depolarizations of body tissue in areas remote from the local sensing site and are manifested as propagated voltage potential wavefronts carried to and incident upon the local sensing site. A far-field signal may comprise the intrinsic signal originating from the chamber of the heart opposite the one in which a lead electrode is located. For example, for a lead electrode implanted in the right atrium, the ventricular R-wave comprises a far-field signal whose amplitude can easily swamp the smaller P-wave signal sought to be sensed thereby making difficult the discrimination of a P-wave from the higher energy QRS complex (sometimes referred to as the R-wave).
Sensing in the coronary sinus also presents far-field signal issues. For example, deep in the coronary sinus where sensing of left ventricle activation would be expected, the right atrium signal may be sensed as a far-field signal. Similarly, in the proximal coronary sinus, where the sensing of left atrial activations would be expected, far-field signals originating in the left ventricle may be extremely strong. Moreover, early right ventricular activation can interfere with sensing of left ventricle signals.
The sensing electrode(s) detect or sense the voltages of these far-field signals and interpret them as depolarization events taking place in the local tissue when such polarizations are above the threshold sensing voltage of the system. When far-field signal voltages greater than the threshold voltage are applied to the sensed signal processing circuitry of the pulse generator or pacemaker, activation of certain pacing schemes or therapies can be erroneously triggered.
With the development of programmable, universal stimulation/sensing systems, that is, three and four chamber combination pacemaker-cardioverter-defibrillators, accurate sensing of cardiac signals has become even more critical, and the management, suppression and/or elimination of far-field signals is vitally important to allow appropriate device algorithms to function without being confused by the undesirable far-field signals.
Approaches to the problem of far-field signal sensing include configuring the circuitry of the pacemaker to attenuate far-field signals, introducing a blanking period long enough to prevent the sensing of unwanted signals, and providing timing and logic circuitry to detect “crosstalk” between paced chambers of the heart and to provide compensation in the event “crosstalk” is detected. These solutions are described in U.S. Pat. Nos. 4,513,752 and 4,825,870 assigned to the owner of the present invention.
U.S. Pat. No. 4,579,119 discloses a tripolar atrial pacing and sensing lead comprising a tip electrode and a pair of spaced-apart ring electrodes. The distal ring electrode, that is, the ring electrode positioned intermediate the tip electrode and the more proximal of the pair of ring electrodes, is at all times connected to one of the two input terminals of a sense amplifier. A multiplexer at the proximal end of the lead selectively connects the remaining electrodes to a pulse generator and to the sense amplifier. Thus, during pacing the tip electrode and the proximal ring electrode are connected to the pulse generator. During sensing, the multiplexer connects the parallel combination of the tip electrode and the proximal ring electrode to the other sense amplifier input. The placement of this tripolar lead in the right atrium is said to reduce both cross-sensing, that is, the sensing of far-field signals originating in the ventricle, and polarization potentials which would otherwise mask the evoked response. The '119 patent, however, requires an electrical conductor extending the entire length of the lead body for each of the electrodes. In addition, the '119 patent does not deal with left heart pacing and sensing.
U.S. Patent Application Publication US2002/0123784A1 discloses a tripolar pacing and sensing lead including three electrodes separated by interelectrode spacings that are said to maximize both sensing and pacing activities. The electrode pair comprising the tip and first ring electrodes provides local sensing capabilities within either the atrium or the ventricle, while the electrode pair comprising the tip and second ring electrodes provides pacing capabilities. Far-field artifacts are said to be virtually eliminated by minimizing the distance between the two sensing electrodes. Like U.S. Pat. No. 4,579,119, the tripolar electrode arrangement of publication US2002/0123784A1 requires a separate electrical conductor running the length of the lead for each electrode and the publication does not address left heart pacing and sensing.